We are investigating the myelinated nerves of the rat kidney, which appear in bundles together with unmyelinated axons in the corticomedullary region, the loose connective tissue around arteries and the subepithelial connective tissue. The electron microscope reveals a close relationship between the unmyelinated and myelinated component of the bundle. The results of light and electron microscopic autoradiography for localization of exogenous tritiated norepinephrine are consistent with the unmyelinated component being monoaminergic. Physioloical studies suggest that the myelinated nerves may be afferent ones. These observations are in keeping with a close functional relationship between the myelinated, possibly afferent and unmyelinated efferent innervation of the kidney. We are currently analyzing in greater detail the distribution of the myelinated innervation of the kidney using light and electron microscopic serial sections. The inneravation of the juxtaglomerular region is being investigated by light and electron microscopic autoradiography for localization of exogenous tritiated norepinephrine. Light microscopy revealed accumulations of grains associated with arterioles and in lesser numbers with the neighboring tubules. Quantitative electron microscopy autoradiography revealed that nearly two-thirds of the silver grains are on axons. Most varicosities, including those in contact with smooth muscle, juxtaglomerular granular or tubular cells, are labeled. These findings are consistent with the glomerular arterioles being innervated mainly be adrenergic nerves.